The present invention relates to a hip joint prosthesis and more particularly to the femoral component of such a prosthesis.
Many methods and devices have been developed to improve the fixation of hip joint prostheses including the femoral component thereof in the body so that the device implanted therein becomes as permanent as possible. Many orthopeadic implants use a cement to anchor the stem portion of a femoral component in the femur. For example, United Kingdom Patent Specification No. 1,409,054 in the names of Robin S. M. Ling (one of the co-inventors of the present application) and Alan J. C. Lee (incorporated herein by reference) discloses a hip joint prosthesis having a double-tapered stem which, among other advantages, enhances extrusion of cement caused by penetration of the stem during fixation. U.S. Pat. No. 3,793,650 (incorporated herein by reference) one of the co-inventors of which is also Robin S. M. Ling, discloses an intramedullary stem for a prosthetic bone joint device having a base with spring members intended to centralize the position of the stem in the canal or bore of the bone in order to insure a relatively uniform or, at least minimum, thickness of cement between the wall of the bone and the stem. It is desirable that there be at least two millimeters (2 mm) of cement between the stem and the bone. By providing a means for insuring that there will be at least a certain minimum thickness of cement between the stem of the prosthesis and the interior wall of the canal formed in the femur bone for receiving such stem, the likelihood of the stem protruding through the cement and contracting the interior of the femur bone itself is minimized. Thus, in those types of implants using cement, it is important to insure that the stem is completely encapsulated by the cement and does not protrude through to contact the bone.
One type of bone cement utilized to retain the stem of a femoral hip joint prosthesis in the canal of a bone comprises a mixture of polymethylmethacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer and optionally including a styrene co-polymer of PMMA. This and other types of cement utilized for such purpose may be packaged in two separate components which are mixed into a paste which is placed in the canal of the femur immediately prior to insertion of the stem of the prosthesis. Such paste then sets to a relatively rigid material providing excellent adherence to the interior wall of the bone. Some prior art femoral stem prostheses have utilized a precoating of a film of PMMA cement. Prostheses having a film of PMMA or other type of bone cement coated thereon prior to implantation are disclosed in the following U.S. Pat. Nos. 4,281,420; 4,336,618 and 4,491,987 (incorporated herein by reference).
Prior to the invention disclosed in our co-pending application Ser. No. 07/527,298 filed May 23, 1990, it had been the belief that it is desirable to have good adhesion between the stem and the cement. Many prior art devices were specifically directed to providing a design for the prosthesis intended to maximize adhesion between it and the cement. This has been true of ones utilizing a precoating as well as ones in which the stem has a bare metallic surface which comes into direct contact with PMMA or other bone cement deposited in the medullary canal of the femur.
In U.S. Pat. No. 4,281,420 and its divisional U.S. Pat. No. 4,336,618, which relate to prostheses utilizing a precoating of film of PMMA, it is pointed out that those inventions are directed towards "prostheses adapted to maximize strength and durability of the prostheses/bone cement adherence". (See "Field of the Invention" for both of the above patents). These inventions are specifically directed to a procedure and device for maximizing the strength of the interface between the prosthesis and the bone cement through the use of a film coating of PMMA having a thickness which may be as thin as 0.0001 inch and preferably is about 0.0002 inch.
U.S. Pat. No. 4,491,987 states as one of the objects that the invention described and claimed therein "improves the interfacial bond between the prosthesis and the bone tissue to which it is affixed." (See column 2, lines 66-68). Under the teachings of that patent, the prosthesis is precoated with a polymer layer that is compatible with the bone cement utilized during surgical implantation and has a maximum thickness controlled only by the size of the bone cavity. The purpose of such layer is to ". . . achieve a structurally sound bond therebetween." The specification goes on to state that "the surface of the prosthesis to receive the polymer coating should be pretreated immediately prior to application of the coating to roughen or otherwise prepare the surface to achieve a structurally sound bond at the prosthesis coating interface." (See column 3, lines 31-36). This patent also explained the great advantages of utilizing a prosthesis having a roughened surface in order to obtain greater "interfacial implant-precoat maximum stress." (See column 9, lines 15-16).
Examples of non-coated prostheses include the CML Cemented Medullary Locking Hip System manufactured by DePuy Division of Boehringer Mannheim Corporation, Warsaw, Ind., which is a hip system in which the upper portion of the stem is provided with a roughened textured surface intended to enhance the bond of the cement to the prosthesis at the prosthetic interface. It also utilizes a "Macro-Textured" waffle design which is intended to increase the surface area and the mechanical interlock between the cement and the prothesis in the area of such waffle design.
Osteonics Corp., Allendale, N.J., manufactures the OMNIFLEX Femoral System of a titanium alloy having a normalized surface to promote good adhesion of the cement thereto.
Other types of devices which disclose the use of cement within a bore or canal of the femur are described in U.S. Pat. Nos. 3,829,904; 3,874,003; 4,012,796 and 4,080,666. The disclosures of the aforesaid patents are hereby incorporated by reference. Copies of such patents are enclosed.
In cemented types of devices used heretofore, both those having PMMA or other types of precoatings and those without any such precoating, problems have arisen, particularly after a number of years of implantation. Problems have also arisen with femoral hip joint prostheses which do not utilize any bone cement for implantation. With respect to the cemented type devices, part of the problem arises from the fact that the cement utilized to retain the stem of the device in the canal of the femur bone is subject to a phenomenon known as creep. Thus, while the bone cement appears to be rigid when set, it is subject to minute amounts of movement over time. The amount of creep encountered with such cement following implantation is exaggerated by virtue of the fact that the body temperature controls the temperature of the implanted cement and prosthesis. Thus, PMMA and other types of bone cement at body temperature are subject to a greater degree of creep than bone cement maintained at room temperature of, say, 72.degree. F. This may be readily observed by mounting a bar of PMMA so that its ends are supported and applying a fixed load at the center of the bar. Tests have shown that a bar so supported and subjected to a load of 5 pounds for eight hours at 98.6.degree. F. will deflect to an extent 3.5 times greater than an identical bar supported and loaded in an identical manner for eight hours at 72.degree. F.
Over a period of time, the phenomenon of creep may result in disruption of the micro-interlocking of the cement-bone interface, especially if the cement mantle is firmly bonded to the femoral prosthesis. As is well known in the field of hip replacements, it is important that there be a good bond between the cement and the bone and that there be no disruption in the micro-interlocking of the cement-bone interface.
Subsidence of the femoral component occurs in various degrees with prostheses of different designs regardless of the presence or absence of collars. Any prosthesis which is firmly bonded or fixed to the cement will, upon subsidence, disrupt the cement bone interface and inevitably lead to clinical loosening and subsequent failure necessitating revision. The presence of a collar may cause the stem of the prosthesis to go into varus during any such subsidence.